A membrane electrode assembly (MEA) may be the central element of electrochemical devices such as proton exchange membrane fuel cells, sensors, electrolyzers, chlor-alkali cells, and the like. Such MEAs typically comprise an ion conductive membrane (ICM), which functions as a solid electrolyte, in contact with electrode layers that include catalytic electrode material such as platinum. In a typical electrochemical cell, an ICM is in contact with a cathode layer and an anode layer, and transports ions that are formed at the anode to the cathode, allowing electrical current to flow in an external circuit connecting the electrodes.
One form of catalyst used in MEAs consists of Pt or Pt alloys coated onto carbon particles by wet chemical methods, such as the reduction of chloroplatinic acid. This conventional form of catalyst is dispersed with ionomeric binders, solvents and often polytetrafluoroethylene (PTFE) particles, to form an ink, paste or dispersion that is applied to either the ICM or to an electrode backing material to be placed adjacent to the ICM. In addition to providing mechanical support, it is generally believed in the art that carbon support particles provide necessary electrical conductivity within the electrode layer.
In another variation, Pt fines can be mixed directly with a solution of solvents and polymer electrolyte or Teflon.TM. and coated onto the electrode backing layer or membrane ICM. However, because of limitations on how small the fines can be made, this approach typically results in very high loading of the catalyst with resulting increase in expense.
Nanostructured composite articles are disclosed in U.S. Pat. Nos. 4,812,352, 5,039,561, 5,176,786, 5,336,558, 5,338,430, and 5,238,729. U.S. Pat. No. 5,338,430 discloses that nanostructured electrodes embedded in solid polymer electrolyte offer superior properties over conventional electrodes employing metal fines or carbon supported metal catalysts, including more efficient use of the electrode material and enhanced catalytic activity per unit mass of Pt.
U.S. Pat. No. 5,879,828 concerns MEAs having electrode layers comprising nanostructured elements. U.S. Pat. No. 5,879,827 concerns nanostructured elements bearing nanoscopic catalyst particles which may be suitable for use in MEAs.